The present invention relates to an apparatus for storing/restoring holographic data and a method for coding/decoding holographic data; and, more particularly, to an apparatus for storing/restoring holographic data and a method for coding/decoding holographic data, which improve a BER (bit error rate) by preventing isolated xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d phenomena from occurring in pixels of holographic data stored to or restored from a storage media.
Recently, many researches on technologies for storing volume of holographic data have been under way by the help of a remarkable development of, e.g., a semiconductor laser, a CCD (charge coupled device), an LCD (liquid crystal display) and so on. Results of these researches are now being applied in such a field as a finger print recognition system, as well as any fields that take advantage of a large-scaled storage capability and a high-speed transmission rate.
A conventional holographic data storing/restoring apparatus stores an interference pattern, which is generated when an object beam for a target object interferes with a reference beam, into a storage media sensitive to an amplitude of the interference pattern, the storage media being made of, e.g., crystal. The apparatus stores data on an amplitude and a phase of the object beam by varying an angle of the reference beam, such that a three-dimensional shape of the target object can be represented on the storage media. In this way, the storage media can contain several hundreds or thousands of holograms, each of which is represented in the form of a page including binary digits.
In a recording mode, the conventional holographic data storing/restoring apparatus splits a laser beam from a light source into a reference beam and an object beam. And then, the object beam is modulated into binary data comprising pixels, each of which represents either light or shade of a target object, according to externally inputted data (i.e., input data to be stored). Next, an interference pattern is obtained through the interference of the modulated object beam (i.e., a signal beam) with the reference beam split from the laser beam and reflected from a mirror having a certain inclination. The interference pattern is stored in the storage media as holographic data corresponding to the input data.
In this case, the holographic data are multiplexed before being stored in the storage media. The multiplexing of the holographic data can be performed by using, e.g., an angle multiplexing, a wavelength multiplexing or a phase multiplexing.
In the meanwhile, in a reproducing mode, the conventional holographic data storing/restoring apparatus shuts out an object beam split from a laser beam. On the other hand, the apparatus irradiates a reference beam split from the laser beam to the storage media after the reference beam being reflected from a mirror at a predetermined angle. Thereafter, the irradiated reference beam is diffracted by employing the interference pattern stored in the storage media, such that a page of demodulated binary data can be obtained.
In this case, the reference beam, which is used in restoring the holographic data, has the same reflection angle as that of the reference beam used in storing the holographic data.
However, the conventional holographic data storing/restoring apparatus has a problem that, in general, an amplitude of a restored signal has a non-uniform distribution due to several factors such as a non-uniformity in an intensity of a laser beam, a distortion noise generated through a lens, and a scattering and a diffraction in the apparatus.
Meanwhile, a conventional method for coding/decoding holographic data uses a threshold value to distinguish between two binary digits, i.e., 0 and 1 when restoring holographic data stored in the storage media. The threshold value can be a fixed value, e.g., 0.5 or an average of pixels included in a page representing holographic data, or a local threshold that is locally determined.
In case the fixed value is used in decoding holographic data, a pixel included in the page is considered to be 1 when a level of the pixel is larger than a threshold value, i.e., the average or 0.5. On the other hand, the pixel is considered to be 0 when the level is smaller than the threshold value. The conventional coding/decoding method, which uses a fixed value as the threshold, is advantageous in that a high code rate can be obtained. However, it has a problem that a restoring error rate becomes higher for decoding data on an edge part of the page.
Meanwhile, the local threshold is determined as follows. First, a page representing holographic data is divided into several areas. And then, a different threshold value is set for each of the areas. That is, a higher threshold value is set for an area near the center of the page while a smaller threshold value is set for an area near the edge of the page. A pixel included in the page is considered to be 0 or 1, respectively, depending on a level of the pixel being smaller or larger than the threshold value.
In case the local threshold is used in decoding holographic data, both a high code rate and a low restoring error rate can be obtained. However, the restoring error rate may increase when an identical set of local thresholds is applied to coding/decoding systems, each of which has a different noise pattern depending on characteristics of the system and other environmental factors.
Alternatively, as a holographic data coding/decoding method for reducing the restoring error rate, a binary differential coding/decoding method can be used. The binary differential coding/decoding method encodes a sequence of binary digits by taking advantage of a characteristic that a level of a pixel representing a binary digit xe2x80x9c1xe2x80x9d is larger than that of a pixel representing a binary digit xe2x80x9c0xe2x80x9d in a local area. For example, binary digits xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d are encoded into xe2x80x9c01xe2x80x9d and xe2x80x9c10xe2x80x9d, respectively. Further, decoding of the encoded binary digits is performed in reverse order of the coding process.
The conventional binary differential coding/decoding method is advantageous in that a low restoring error rate can be obtained, while it is disadvantageous in that a code rate is considerably low (50%).
Further, when binary digits are encoded by using the conventional binary differential coding/decoding method, as shown in FIGS. 5A and 5B, isolated xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d phenomena may occur in a part of a page. These phenomena may cause noises on the page stored in a storage media, which is due to a fact that the object beam modulated in accordance with the encoded binary digits is diffracted while passing through the spatial light modulator.
In this case, a portion of a laser beam corresponding to the isolated xe2x80x9conxe2x80x9d spreads over its neighboring pixels, such that a laser beam intensity corresponding to the neighboring pixels increases while that corresponding to the isolated xe2x80x9conxe2x80x9d pixel decreases. Further, a portion of the object beam corresponding to neighboring pixels of the isolated xe2x80x9coffxe2x80x9d spreads over the page, such that a laser beam intensity corresponding to the neighboring pixels decreases while that corresponding to the isolated xe2x80x9coffxe2x80x9d pixel increases.
In this situation, the isolated xe2x80x9conxe2x80x9d pixel introduces relatively low laser beam intensity compared to a non-isolated xe2x80x9conxe2x80x9d pixel. Also, the isolated xe2x80x9coffxe2x80x9d pixel introduces relatively high laser beam intensity compared to a non-isolated xe2x80x9coffxe2x80x9d pixel. Thus, a laser beam intensity for the isolated xe2x80x9coffxe2x80x9d (or xe2x80x9conxe2x80x9d) pixel may be considered to be higher (or lower) than those for neighboring xe2x80x9conxe2x80x9d (or xe2x80x9coffxe2x80x9d) pixels, which causes a restoring error rate to be higher.
It is, therefore, an object of the present invention to provide an apparatus for storing/restoring holographic data and a method for coding/decoding holographic data that improve a restoring error rate by removing isolated xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d pixels in a page representing the holographic data.
In accordance with one aspect of the present invention, there is provided an apparatus for storing/restoring holographic data, comprising: a light source for generating a laser beam; a beam splitter for splitting the laser beam into a reference beam and an object beam; a first grouping block for dividing input data into a plurality of groups, each of which includes two binary digits; a coding block for encoding each of the groups into a 2xc3x972 data block wherein a first row of the 2xc3x972 data block has the two binary digits and a second row of the 2xc3x972 data block has another two binary digits generated by applying a binary differential calculation on the two binary digits included in the first row; a modulator for generating a signal beam by modulating the object beam in accordance with the 2xc3x972 data block; a storage media for storing an interference pattern generated through an interference of the reference beam with the signal beam; a second grouping block for dividing a signal restored from the storage media into a plurality of 2xc3x972 data blocks; and a decoding block for decoding each of the 2xc3x972 data blocks outputted from the second grouping block into two binary digits by comparing values of a first row of the 2xc3x972 data block with those of a second row of the 2xc3x972 data block.
In accordance with another aspect of the present invention, there is provided a method for coding holographic data, comprising steps of: (a) dividing input data into a plurality of groups, each of which includes two binary digits; and (b) encoding each of the groups into a 2xc3x972 data block wherein a first row of the 2xc3x972 data block has the two binary digits and a second row of the 2xc3x972 data block has another two binary digits generated by applying a binary differential calculation on the two binary digits included in the first row.
In accordance with still another aspect of the present invention, there is provided a method for decoding holographic data, comprising steps of: (a) dividing a signal restored from a storage media into a plurality of 2xc3x972 data blocks; and (b) decoding each of the 2xc3x972 data blocks into two binary digits by comparing values of a first row of the 2xc3x972 data block with those of a second row of the 2xc3x972 data block.